This invention relates to processes for the conversion of carbohydrates to polyhydric alcohols. More particularly, this invention relates to processes for the production of polyhydric alcohols from carbohydrates using a ruthenium metal loaded zeolite catalyst.
The term "carbohydrate" as used throughout the specification and claims includes monosaccharides and polysaccharides. This term includes both pure compounds, such as glucose and sucrose, and mixtures such as corn starch hydrolyzate, which is a hydrolysis product of corn starch containing glucose (dextrose) and oligomers thereof.
The term "polysaccharide" as used in the specification and claims includes those saccharides containing more than one monosaccharide unit. This term encompasses disaccharides and other saccharides containing a small number of monosaccharide units, which are commonly known as oligosaccharides.
The term "conversion" as used herein refers to hydrogenation when applied to monosaccharides and to a combination of hydrogenation and hydrolysis when applied to polysaccharides.
Catalytic processes may be broadly divided into processes using heterogeneous catalysts and those using homogeneous catalysts. Heterogeneous catalysts are those which are insoluble in the reaction medium, and are typically solid materials. Homogeneous catalysts are those which are soluble in the reaction medium, and are typically liquid. This invention is concerned with processes using a heterogeneous catalyst.
The conversion of carbohydrates to polyhydric alcohols using ruthenium on a solid carrier is known. U.S. Pat. No. 2,868,847 discloses the use of ruthenium on an inert catalyst support such as carbon, alumina, silica, or kieselguhr as a catalyst for the catalytic hydrogenation of saccharides such as dextrose, levulose, sucrose, maltose, and lactose. Starting materials include monosaccharides, e.g. dextrose and levulose, and disaccharides, e.g. sucrose, lactose, and maltose. Dextrose was hydrogenated to sorbitol and sucrose and lactose were hydrolyzed and hydrogenated to hexitols. However, maltose, a disaccharide containing two glucose units, was more easily converted to maltitol, a C.sub.12 alcohol, according to the patent.
The supported nickel catalysts described in U.S. Pat. Nos. 3,538,019 and 3,670,035 (which is a division of U.S. Pat. No. 3,538,019) have high activity for the conversion of both monosaccharides and polysaccharides, including carbohydrate mixtures such as corn starch hydrolyzate, with high selectivity to sorbitol when either corn starch hydrolyzate or dextrose is used as the starting material. Carbon, diatomaceous earth, and kieselguhr are disclosed as carriers. This represents a significant improvement over the process and catalyst of U.S. Pat. No. 2,868,847, since the relatively inexpensive corn starch hydrolyzate, or other commercially available carbohydrate mixtures, can be used as the starting material in place of the much more expensive pure sugars. A disadvantage of the catalyst in U.S. Pat. Nos. 3,538,019 and 3,670,035 is that the catalyst cannot be regenerated; when reactivation is required, it is necessary to remove the active catalyst material from the support by chemical means and then to redeposit the catalyst metal on the support. Various other nickel catalysts for conversion of carbohydrates to polyhydric alcohols are cited in U.S. Pat. Nos. 3,538.019 and 3,670,035.
The hydrogenation of monosaccharides using a supported ruthenium, palladium, platinum, or nickel catalyst (activated carbon was used as the support in all experimental work) is discussed in an article by N. A. Vasyunina et al., "Catalytic Properties of Ruthenium in Monosaccharides Hydrogenation Reaction", in Izvestiya Akademii Nauk SSR Khimicheskaya Seriya 4:848-854 (1969). Ruthenium was found to have higher activity than the other three catalysts.
A two stage process for hydrogenation of ligneous and other plant material such as wood sawdust is disclosed in Izv. Akad Nauk SSR, Otd. Khim. 8: 1522-1523 (1960). The process consists of a first stage hydrolytic hydrogenation of polysaccharides in an acid medium, followed by a second stage hydrogenation of the lignin in an alkaline medium, using a ruthenium catalyst in both stages. In a specific embodiment, pine sawdust is treated using an aqueous phosphoric acid medium and a ruthenium on carbon catalyst. The first stage reaction product is filtered to separate the liquid medium from the crystals obtained from the first stage filtrate.
The use of solid ruthenium-containing zeolite catalysts for hydrogenation reactions other than the hydrogenation of carbohydrates is also known. U.S. Pat. Nos. 3,200,082 and 3,200,083 disclose catalysts comprising noble metals including ruthenium on zeolites X, Y, and L. U.S. Pat. No. 3,767,720 discloses the hydrogenation of aromatic hydrocarbons (e.g., benzene) to cyloolefins using various catalysts including ruthenium on 3A, 4A, 10X, and 13X molecular sieves. Other references disclosing ruthenium-containing zeolite catalysts include U.S. Pat. Nos. 3,197,398; 3,239,451; 3,269,934; 3,324,047; 3,364,135; 3,375,205; 3,459,676; 3,476,821; 3,524,809; 3,600,301; and 3,647,681.
Zeolites A and X are essentially crystalline synthetic aluminosilicate zeolites having a silica/alumina mol ratio less than 3. Zeolite A is described and claimed in U.S. Pat. No. 2,882,243 and zeolite X is described and claimed in U.S. Pat. No. 3,882,244, both issued to Union Carbide Corporation. Zeolites Y and L are essentially crystalline synthetic aluminosilicates having silica/alumina mol ratios greater than 3, i.e., from 3 to about 6 in zeolite Y, and from about 5.2 to about 6.9 in zeolite L.
Zeolite Y (sodium form) is described in U.S. Pat. No. 3,130,007, and decationized zeolite Y is disclosed in U.S. Pat. No. 3,130,006, also both issued to Union Carbide Corporation. Zeolite L. is described and claimed in U.S. Pat. No. 3,216,789, also issued to Union Carbide. Some more recent references, such as U.S. Pat. No. 3,324,047 and G. T. Kerr, "Molecular Sieves", Advances in Chemistry Series No. 121 (American Chemical Society), pages 219-228 (1973), state that "decationized zeolite Y" and "Hydrogen zeolite Y" (i.e., the hydrogen form of zeolite Y) are the same. Zeolites are ordinarily synthesized in the sodium form; other metallic or ammonium ions can be introduced into the zeolite by ion exchange, as is well known in the art. Zeolites in the hydrogen form can be obtained by decomposition of the ammonium form at high temperature, according to methods known in the art. The structure of zeolites is discussed extensively in D. L. Breck, "Zeolite Molecular Sieves", published by John Wiley and Sons, New York, 1974. A comprehensive review of zeolite catalysts is contained in an article by J. Turkevich, Catalysis Reviews, 1, 1-35 (1967).
Although various catalytic processes for the conversion of carbohydrates to polyhydric alcohols are known in the art, none possesses all of the attributes which are desirable in such processes, e.g., ability to use inexpensive mixed carbohydrates; high selectivity to sorbitol when either glucose or a starch hydrolyzate is used as the starting material; high catalyst attrition resistance; and ease of catalyst regeneratability.